In this paper, a reflector antenna design method is proposed to improve transmission efficiency and control output intensity distribution. Primary and secondary mirrors are constructed segment by segment using a numerical method according to the law of energy conservation, the vector theory of reflection, and Fermat's principle that the optical path length of each ray must be equal. To verify the effectiveness and practicability of this method, an example of an antenna with the prescribed intensity distribution of the output beam is given. The transmission efficiency of the antenna system designed by this method reaches 100% in theory. The effects of output intensity distribution and deviation on transmission efficiency were also analyzed. We believe that the results show that the proposed method is a highly efficient approach to the design of a freeform reflector antenna to improve transmission efficiency and control the output intensity distribution.